Melt butter in a Dutch oven over medium-high heat. Add shallots and garlic to pan; sauté 1 1/2 minutes. Add saffron and tomatoes; cook 2 minutes. Stir in wine, parsley, pepper, and juice; bring to a boil. Cook 4 minutes. Add mussels. Cover and cook 6 minutes or until shells open; discard any unopened shells. Serve over pasta.